Science in Everyday Life — Class 3 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 3 Science. Kitchen science, simple machines and staying safe.
1. Science in the kitchen
Much of what happens in a kitchen is science you have already met.
Boiling turns a liquid into a gas. Water becomes steam at 100 degrees Celsius.
Freezing turns a liquid into a solid. Water becomes ice at 0 degrees Celsius.
Fermentation is the change that makes idli and dosa batter rise. Tiny living organisms in the batter produce a gas, and the bubbles of that gas lift the batter and make it light. Fermenting also makes the food easier to digest.
2. How a pressure cooker works
A pressure cooker is sealed tightly, so the steam cannot escape.
As steam builds up, the pressure inside rises. Under higher pressure water boils at a temperature above 100 degrees Celsius, and hotter water cooks food faster. That is the whole idea: less time and less fuel.
The weight on top lifts and whistles when the pressure gets too high, letting a little steam out. That whistle is a safety valve, not a timer.
Never open a cooker straight after switching off the flame. It is still full of pressure and very hot steam, and opening it then can cause a bad burn. Wait until the pressure has dropped and the weight lifts freely.
3. Simple machines around the house
A simple machine makes work easier.
| Machine | What it is | Examples at home |
|---|---|---|
| Lever | A stiff bar turning about a fixed point | Lemon squeezer, bottle opener, scissors, seesaw |
| Wedge | A triangular tool that splits things | Knife, axe, nail |
| Inclined plane | A slope instead of a step | Staircase, ramp |
A pair of scissors is two levers joined at the point where they turn, with a wedge on each blade.
4. Safety in the kitchen
- Hold hot handles with a dry cloth, never with a bare hand.
- Turn pot handles inward, so they cannot be knocked as you pass.
- Keep paper, cloth and plastic well away from the flame.
- Do not run in the kitchen.
5. Safety at home
- Never touch a switch or an appliance with wet hands. Water conducts electricity, so wet hands give the current a path through your body.
- Never fly a kite near power lines. A wet or metallic string can carry current down to you.
- Keep floors dry, since a wet floor is where most household falls happen.
- Never mix cleaning liquids. Two harmless-looking bottles can produce a poisonous gas when combined.
6. Worked examples
Example 1. Why does food cook faster in a pressure cooker?
The sealed cooker traps steam, so pressure rises. Under higher pressure water boils above 100 degrees Celsius, and hotter water cooks faster.
Example 2. What makes idli batter rise?
Fermentation. Tiny organisms in the batter give off a gas, and its bubbles make the batter light.
Example 3. Which simple machine is a lemon squeezer, and which is a knife?
The lemon squeezer is a lever, turning about a fixed point. The knife is a wedge, splitting things apart.
Example 4. Why is it dangerous to touch a switch with wet hands?
Water conducts electricity. Wet hands complete a path for current to pass through the body, which can give a severe shock.
7. Practice
- Name the change of state that happens when water boils, and when it freezes.
- What causes idli batter to rise?
- Explain in one sentence why a pressure cooker cooks faster.
- What is the whistle on a pressure cooker for?
- Name one lever, one wedge and one inclined plane from your home.
- Give two kitchen safety rules.
- Why should you never mix two cleaning liquids?
8. Answers
- Boiling is liquid to gas; freezing is liquid to solid.
- Fermentation, in which tiny organisms produce a gas whose bubbles lift the batter.
- Trapped steam raises the pressure, so water boils above 100 degrees Celsius and the hotter water cooks food faster.
- It is a safety valve that releases steam when the pressure inside gets too high.
- Lever: bottle opener. Wedge: knife. Inclined plane: staircase or ramp.
- Hold hot handles with a dry cloth, and keep cloth and paper away from the flame.
- Because the two together can produce a poisonous gas, even if each is safe on its own.
9. Summary
- Boiling, freezing and fermentation are everyday examples of science in the kitchen.
- Fermentation makes idli and dosa batter rise and easier to digest.
- A sealed cooker raises the pressure, so water boils hotter and food cooks faster.
- The cooker's whistle is a safety valve; never open a cooker while it still holds pressure.
- Levers, wedges and inclined planes are simple machines found all over a house.
- Never touch electrical things with wet hands, since water conducts electricity.
- Never mix cleaning liquids: the combination can give off a poisonous gas.
